1. Field of the Invention
The present invention relates to a filter for outputting the power of a signal passing through a specified frequency band for each frame (short time division).
2. Description of the Prior Art
In spectrum analysis and sound identification, FFT (fast fourier transform) and digital filter are two typical means for obtaining the power of a signal of a particular frequency band through digital processing.
The method that uses FFT is described below. First, a signal waveform is operated upon by the FFT to calculate the power of each frequency component. Then, the powers of the frequency components corresponding to the passing band of the filter are multiplied by the frequency characteristic of the filter and summed up, so that the power for each frame is obtained. This method is advantageous in that a filter of a desired characteristic is easily realized. The problem is, however, that FFT involves many calculation stages.
The method using a digital filter is described below. After a signal is passed through a specified filter, the square-law detection is conducted on the waveform. Then, the signal is passed through a low-pass-filter to sample the power for each frame. Referring to FIG. 1, an analog signal 101 is converted by an analog/digital converter 102 to a digital signal 103 which is passed through a digital filter 104 with a specified frequency characteristic so as to obtain a signal 105 having the passing band frequency component. Then, the signal 105 is passed through a square-law detector 106 to square the signal value, thus obtaining an instantaneous power signal 107. The signal 107 is then passed through an anti-alias low-pass-filter 108 to obtain a reflected noise-free signal 109 which is sampled by a sampling circuit 110 at frame intervals to obtain output power 111.
Though this digital filter method involves fewer calculation stages, it is necessary to increase the number of degrees of the digital filter to improve the blocking characteristic of the filter, resulting in increased calculations after all. In addition, calculation stages increase in proportion to the number of filter channels.